Field of the Invention
The present invention concerns a method to quickly determine regions of modified temperature in sample volume by magnetic resonance tomography using a multi-echo sequence, as well as a magnetic resonance tomography apparatus to implement such a method.
Description of the Prior Art
In magnetic resonance measurements, the interaction of magnetic moments of atomic nuclei (the nuclear spins) with an external magnetic field is detected. The nuclear spins are aligned in an external basic magnetic field and precess with the Larmor frequency (which depends on the value of the magnetic moment of the atomic nuclei and the external magnetic field) around the axis of the alignment in the magnetic field, after excitation by an alternating external electromagnetic (radio-frequency) field. The atomic nuclei thereby generate an alternating electromagnetic field at the Larmor frequency. The initial amplitude, the phase coherence of this precession, and the decay of the excitation, change depending on the temperature of a sample in which the atomic nuclei are located. A temperature change of the sample between the two measurements can be determined by comparison of the two measurements.
For example, from EP 534607 A1 it is known to employ magnetic resonance tomography with a diffusion-sensitive imaging method, in order to monitor a medical treatment that makes use of heat. The local temperature is linked with the local diffusion rate and a temperature change can be depicted by imaging (mapping) of the temperature change. Even with the fastest known sequences, a complete three-dimensional acquisition requires a certain time that, due to different physical effects such as relaxation times, cannot be shortened without loss of sensitivity.
In the acquisition of magnetic resonance data, the frequency of the radio-frequency excitation signals has increased with the use of ever stronger basic magnetic fields, up to 3 T or more, with which the specific absorption rate SAR in turn increases quadratically. The measurement time can be limited by SAR regulations that set the maximum allowable heating of the body of the patient by the radio-frequency signals. In particular, non-homogeneous distributions of the field strengths in the patient (as can occur particularly given use of local transmission coils) are critical and can lead to a local overheating of and damage to tissue, in spite of complying with the limit values for the entire body. Furthermore, it is a problem that locally strictly limited temperature elevations cool quickly due to the distribution of thermal energy by diffusion or blood flow, and thus in a slow measurement are not recognizable in a form that indicates the risk such temperature elevations present to damaging tissue.
Various methods are also known to destroy pathological tissue by means of hyperthermia or ablation. In addition to completely killing the pathological tissue, care must be taken to preserve the surrounding tissue. This can be achieved by heating the pathological tissue optimally quickly and briefly to or above the critical temperature. Due to the heat propagation into the surrounding tissue, it is also necessary to measure the temperature thereof in very short time intervals.